The present invention relates to heat-sensitive recording materials and more particularly to heat-sensitive recording materials capable of retaining record images with a high stability.
Heat-sensitive recording materials are well known which make use of the color forming reaction of a colorless or pale-colored basic dye with an organic or inorganic color developing material which develops a color on contact with the dye, such that the two materials are brought into contact with each other by heating to produce a color image. Since the above heat-sensitive recording materials are relatively inexpensive and record images are formed thereon with use of a compact printing device with an easy maintenance, the recording materials are not only used as recording media for a facsimile, various calculators or the like but also used for other various purposes.
For example, such heat-sensitive recording materials are being used as heat-sensitive labels increasingly with the development of POS system (POS=point of sales) for retailers.
If the POS system is introduced into a supermarket and the like, such heat-sensitive labels used are frequently brought into contact with water, wrap films (and plasticizers contained therein), oils and the like with the result that the record images on the labels would be reduced in color density on contact therewith. To obviate this problem, it is desired to provide heat-sensitive recording materials having sufficient resistance to water, plasticizers, oils and the like.
To improve the ability of recording materials to retain record images, proposed are a method of coating a heat-sensitive recording layer with an aqueous emulsion of a resin having a film-forming property and resistance to chemical substances (Japanese Unexamined Patent Publication No. 128347/1979), a method of coating a heat-sensitive recording layer with a water-soluble high-molecular weight compound such as polyvinyl alcohol (U.S. Pat. No. 4,370,370), a method wherein a heat-sensitive recording layer is covered with a protective layer predominantly containing carboxyl group-modified polyvinyl alcohol and then treated with a metal ion solution (Japanese Examined Patent Publication No. 39078/1983), etc. However, the proposed methods accompany new defects together with the improvements and satisfactory effects are not invariably obtained.
For example, in case a coating of an aqueous emulsion of a resin or water-soluble high-molecular weight compound or like binder is formed over a heat-sensitive recording layer, it is necessary to limit the drying temperature to avoid undesirable color formation (fogging) on the recording layer due to drying at high temperatures. However, the drying at limited temperatures would lead to insufficient curing of the binder and consequent adhesion of the resulting protective layer to the recording head (hereinafter referred to as "sticking"). Further the recording material would be impaired in the ability to retain record images.
It is possible to accelerate the curing of the binder using an aqueous solution of a metal salt such as aluminum chloride or sulfate, as proposed in Japanese Examined Patent Publication No. 39078/1983. The proposed method, however, lowers the whiteness of heat-sensitive recording material, thereby deteriorating the commercial value thereof.